Diversity receiving system



Aug. 17, 1948. M. G. CROSBY DIVERSITY RECEIVING SYSTEM 2 Sheets-Sheet 1 Filed May 6, 1945 @mi Q.

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INVENTOR MURRAY G. CROSBY BY /wv-(A/ ATTORNEY Aug. 17, 1948. M. G. cRosBY DIVERSITY RECEIVING SYSTEM 2 Sheets-Sheet 2 Filed May 6, 1943 INVENTOR MURRAY @.CROSBY BY ATTORNEY Patented Aug. 17, 1948 DIVERSITY RECEIVING SYSTEM Murray G. Crosby, Riverhead. N. Y., assignor to Radio Corporation of America, a corporation of Delaware 14 Claims. 1

This invention comprises a simplified diversity receiver for switching a common receiving system to that one of a pair of spaced antennas in which the strongest signals appears. V

Diversity receiving systems are employed to overcome the effect of fading, Such diversity systems include a plurality of geographically spaced antennas so separated that the signal fades differently at the different antennas; In using diversity systems, it is known to switch a common receiving circuit from one antenna to another ln order to receive the signal of strongest intensity. One such arrangement is described in Peterson United States Patent 2,243,118, granted May 27, 1941, which describes an arrangement employing a pair of grid-controlled gas tubes to effect the switching action. Other arrangements for achieving the same result have employed mechanicalrelays, The mechanical switcher has the disadvantage of introducing clicks into the receiving circuit.

The present invention provides a simplified and improved electronic switching arrangement for a diversity receiver which is completely electronic and fast in operation,

A more detailed description of the invention follows in conjunction with a drawing, in which Figs. 1 and 2- ii-lustrate two embodimentsA of a -diversity receiver in accordance with the present invention.

Referring to Fig. 1 of the drawing in more detail, there is shown a pair of antennas I and 2 which are separately coupled through radio frequency transformers 3 and I, respectively, to coupling tubes 5 and 6. Antennas I and 2 may 'be geographically spaced from one another as in commonly employed diversity receiving systems, or, alternatively, these antennas may have the same space location but have different polarizations or different vertical angles of reception. 'Ihe output electrodes of both coupling tubes 5 and 6 are connected in parallel by means of a connection 1. Energy from both of these coupling tubes is fed to a tuned transformer 8 in circuit with a suitable radio frequency apparatus 9. This apparatus 9 may be a tuned radio frequency system for supplying amplified radio frequency output, or, if desired, an amplifying and heterodyning system for beating the incoming energy in transformer ili down to an intermediate frequency and supplying output of this intermediate frequency. The output from apparatus 9 is fed to a transformer Ill and rectified by diode Il. The

detected output is availabieinlead I2Y for utili-A zation by a suitable device, such as a loudspeaker,

headset, or recorder throughrthe intermediary of an audio amplifier (not shown) The condenser 34' is provided for enabling the system to supply a radio frequency output if apparatus 9 .is of the tuned radio frequency type, or to supply an intermediate frequency output if apparatus 9 is of the heterodyning type. and this condenser Il is employed only in the event the system of Fig. 1 is utilized in an embodiment of the type shown in Fig. 2 to be described later. Y

The parallel tuned input circuits A and B for tubes 5 and 6, and the-parallel tuned circuits C and D, respectively, associated with the primary and secondary windings of transformer l are all tuned to the same radio frequency which is the frequency of the incoming signal. The parallel tuned circuits E and F, respectively, associated with transformer I0, however, are tuned. to `a lower intermediate frequency when apparatus Q is of the heterodyning type, but tuned to the same radio frequency as the input circuits when appa ratus 9 is a tuned radio frequency system.

Coupling vacuum tubes 5 and 6 receive their bias potentials from the output of an electronic trigger circuit I3, which in turn is controlled by a pentagrid control or modulator tube I5 through an audio transformer Il. The control grid GI of modulator .tube-Ii is connected 'to a suitable source of low frequency energy I6 which, for example, may be cycles and obtainable from the ordinary house power supply. The conductivity of the modulator tube I5 is controlled by means of negative automatic gain control potentials supplied to the grid G3 of tube l5 by means of lead I1 from a time-constant circuit R. C connected to the output resistance I8 of the diode detector Il. Resistor I8 is connected between the.cathode K of tube II and the cathode end of tuned circuit F. A lead. I9 supplies auto matic gain control potentials to the grids of the radio frequency or intermediate frequency amplifier stages of the receiver 8. The electronic trigger circuit I3 is of a well known type and comprises a pair of vacuum tubes whose anodes and grids are interconnected to have two degrees of stability. These tubes are unstable when both are drawing current but stable when one tube is passing current and the other blocking The change from one position of stable equilibrium of the electronic trigger circuit, such as when one tube is blocking and the other tube passing, to the other condition when the first tube is passing and the other tube blocking, is caused by the presence of a predetermined potential on the leads extending to the secondary winding of the transy former Il. Such an electronic trigger circuit is more adequately described in United States Patent 1.844.950, granted to Finch on February 16, 1932, to which reference is made for a more detailed explanation of the operation thereof.

The time-`constant of the R, C circuit confv nectedto-leads Hand Ilissuchastoremove fast variations of potentials and pass slow variacircuit. It will thus be' seen that the eiectront trigger circuit I3, under control of the modulate] tions representative of relatively slow fading of the incoming Signal. These fast variations include the applied modulation frequency.

In the operation of the system, source Ii will normally permit the pentagrid modulator tubel I5 to pass current at the frequency of the source I6.

These low audio frequency currents appearing tube and the output of the receiver I, functions ti render only that coupling tube 5 or 6 conductivl to which is applied the stronger signal, assuming that there is occasional fading on the signals collected by the two antennas, so that the switchini action will be set into operation.

It will be noted that. if both .signals fade ai the same time. tube I5 becomes conductive and low frequency source Il is continuously appliecl to electronic trigger circuit Il.- This causes a conin transformer I4 will cause the electronic trigger vacuum tubes 5 and'I so as to enable them to alternately amplify the incoming signals. Coupling tubes l and I are so arrangedth'at either one or the other will pass current, depending upon which one has its control grid GI more positive than the other, a condition controlled by the electronic trigger circuit I3. Thus, when tube 5 is conductive it 'will pass the signals received from antenna I, and when tube i is conductive it will pass the signals received on antenna 2. The apparatus l will th'us have impressed on its input the signal from only one of the antennas I or 2 'at any one time, and'thisv signal, which is detected by diode I I, will be impressed on automatic gain control lead Il to the time-constant circuit R; C for controlling the pentagrid modulator 'tube Y The manner In which the system functions switch the input of the radio frequency apparatus 9 to only that antenna whichl at the -moment has the strongest incoming signal will now be described: Let it be assumed that antennal has a strong signal. This will be amplified by vtube 5 and apparatus 9 and detected by diode II.

yThis detected output-will be fed as a negative being drawn by tube VI.

trigger circuit I3 will thus remain in its last condition of stable equilibrium, which is th'e condition for enabling coupling tube 5 to amplify and for preventing coupling tube 6 from amplifying.

f If at this time the signal collected by antenna I should fade, the negative bias fed to the grid G3 by lead I1 will be reduced'to such an extent as to permit the modulator tube I5 to pass current and permit the low frequency source IB to again control the electronic trigger circuit Il to change its state of equilibrium. This change in the state of the electronic trigger circuit will now permit vacuum tube 8 to amplify and will bia-s tube 5 to :cut-oil. If a strong signal now appears on antenna 2, it will be amplified in tube 6 and be passed on to receiver 9, andwill serve to control the modulator tube I5 to prevent any change in the state of equilibrium of the electronic trigger tinual alternation of the ampiincations of tubes 5 and t. As'soon as a signal of sumcient strength .appears on either antenna,'the alternation i:

stopped by the application of the rectiiied signal to electronic G3 of tube I5 and the particular antenna furnishing the stronger signal remains in control. The electronic trigger circuit Il and its controlling amplifier I5 thus act as a device which 'tests the strength ofthe signal from each antenna and leaves the receiving apparatus -connected tothe antenna having the strongest signal.

A more detailed description of the operation of the electronic trigger circuit I3 will now be given. Let it be assumed that anode current is This causes a drop in potential in resistor RI in the anode circuit of tube VI. -The drop in potential is such as to make the anode of tube VI more negative so that a negative potential is fed through feed back resistor R to the grid of tube V. This negative potential forms a drop on grid resistor R2 which biases tubeV to cut-off. With tube V'atcut-oil, there 'is no anode current through resistor RI in its anode circuit so that the feed back to the grid of tube VI is at its maximum positive value. All of these conditions are such as to maintain an equilibrium that will not be upset unless there is an opposing force applied by transformer I4. If this transformer.A applies suiiicient positive voltage to the grid of tube V and suilicient negative voltage to the grid of tube VI, tube VI will be cut oil' so that the grid of tube V will be swung positive. This swings the equilibrium to a condition such that tube VI is cut oi and tube V draws current. This equilibrium will be maintained until the opposite opposing voltage from I4 is sufficient to overcome it. Th'us, it will be seen that the circuit triggers from a first condition in which one tube draws current and the other is cut oil', to a. second condition in which the one tube is cut off and the other draws current. Th'e drop across the anode resistor RI in' one of 'the tubes throws a negative voltage onthe grid of one of the radio frequency amplifier tubes 5 or 6 so as to cut, it off. The Aother amplifier tube 6 or 5 is biased to its operating point since there is no current in the anode resistor feeding the vgrid of the amplifier. If the triggering force is removed, the circuit 'remains in' the condition which existed just before the force is removed. This type of action allows the electronic trigger circuit I3 to hunt between the two diversity antennas I and 2 until one of the antennas obtains a strong enough signal to switch on the triggering force and leave the apparatus 9 connected to that antenna :collecting this strong signal.

Fig. 2 shows how units of Fig. 1 may be utilized in a system employing more than two antennas.

Units 2o, 2| and 22 each include the two coupling tubes, the common amplifying and/or heterodyning apparatus, the rectifier, the modulator tube,` and the trigger circuit as illustrated and described in connectionv with Fig. 1. The apparatus 9 of umts 20 and 2| should be of the tuned radio frequency type supplying radio frequency output. The condensers 34 in circuit with the detectors of units 2|! and 2|A enable the radio frequency outputs to be supplied to the unit 22.

In unit 22, however, the apparatus 8 is preferably vof the heterodyning type. With switch S on the upper contacts, three antennas 3|, 32 andl 33 are in use. With switch S on the lower contacts,

four antennas 3|, 32, 35 and 36 areutilized with characteristics, common receiving apparatus.

therefor including a detector', a coupling tube between each antenna andthe input of said re" ceiving apparatus, a trigger circuit for supplying bias potentials to said coupling tubes to render thjfm alternatively conductive, a control tube having van electrode, means coupling the output of said control tube to said trigger circuit for controlling said trigger circuit, a source of low frequency currents couplecito said Aelectrode of said control tube to enable said control tube `t0 pass current at the frequency of said source, and means coupled between said control tube and the detected output of said commonreceiving apparatus for controlling the conductivity 0f said control tube.

2. In a diversity receiving system, a pair of antennas having relatively different fading characteristics, common radio frequency amplifying apparatus therefor, separate tubes coupling said antennas to the input of said amplifying apparatus, and means for rendering only that coupling tube conductive upon which is impressed a strong incoming signal, said means including a trigger circuit biasing said coupling tubes such that only one of them is conductive at any one time, a detector coupled to the output of said radio frequency amplifying apparatus, a modulator tube` having its output circuit coupled to said trigger circuit and its input circuit coupled to a source of low frequency currents, a lead for supplying bias potentials to said modulator tube from said detector, and a time-constant circuit connected between said lead and said detector for removing signal modulations from' said lead. 3. A diversity receiving system comprising a pair of antennas having relatively different fading characteristics, separate coupling tubes for said antennas, said coupling tubes each having input and output electrodes, a radio frequency transformer between each antenna and the lnput electrode of its associated coupling tube, a connection between the output electrodes of said coupling tubes to provide -a common output circuit, radio frequency electron discharge device apparatus having its input coupled to the common output of said coupling tubes, al rectifier in the output of said radio frequency electron discharge device apparatus, a modulator tube having a control electrode, means for supplying negative bias potentials to said modulator tube from said rectifier to thereby control the conductivity of said modulator tube, a source of low frequency current coupled to said control electrode of said modulator tube, an electronic trigger circuithavingv two degrees of electrical stability coupled tothe output of said modulator tube, and connections from the output of said trigger circuitfor supplying bias potentials to said coupling tubes to enable only that coupling tube to pass current which has the strongest sigi y' nal impressed thereon.

4. A diversity receiving system comprising a pair of'antennas .having relatively different fading characteristics, radio receiving apparatus including a rectifier, separate tubes coupling s-aid antennas to said receiving apparatussaid coupling tubes having a common output circuit, and means for rendering that coupling tube conductive upon'which is impressed a-strong signal, said means including a modulator tube having a control grid, a source of low frequency currents coupled to the control grid' of said modulator tube, a circuit for controlling the conductivity of said modulator tube from the rectified output of said receiving apparatus, and a circuit from the output of said' modulator tube for supplying dii'- ferent bias potentials to said coupling tubes.

5; A diversity receiving system comprising a pair of antennas having relatively differentfading' characteristics, radio receiving apparatus including a rectifier, separate tubes coupling said antennas to said receiving apparatus through separate radio frequency transformers, said cou- 'plng tubes having a common output circuit, and means for rendering that coupling -tube conductive upon which is impressed a strong signal, said means including a modulator tube having a control grid, a source of low frequency currents coupled to the control grid Vof said modulator tube, a circuit for controlling the conductivity of said modulator tube from the rectied output of said receiving apparatus, and an electronic trigger circuit under control of the output of said modulator tube for supplying different bias potentials to said coupling tubes.

6. A diversity receiving system comprising a pair of antennas having relatively different fading characteristics, a radio frequency transformer for each antenna, a coupling vacuum, tube in the output of each transformer, a common output circuit for said coupling tubes, heterodyning apparatus having its input coupled to said common output circuit, a rectifier for said heterodyning apparatus, a multi-grid tube, a connection including a time-constant circuit for supplying said multi-grid tube with bias potentials from said rectifier, whereby the conductivity of said multigrid tube is controlled by the resultant rectified currents, a source of sixty-cycle current coupled to a grid of said multi-grid tube, an electron trigger circuit having two degrees of electrical stability controlled by the output of said multigrid tube, and connections from said trigger circuit for supplyingbias potentials to said coupling tubes, whereby only that coupling tube is rendered conductive which has a strong signal impressed thereon by its associated antenna in the presence of fading of the signal at the other 7 4 tube from the rectified output o f said receiving apparatus, a source of low frequency currents coupled'to a grid of said multi-gridtube. and an electronic trigger circuit under control of the output of said multi-grid tube for supplying difl ferent bias potentials to said coupling devices.

8. The combination with a first fading control system comprising a pair of antennas having relatively different fading characteristics, radio receiving apparatus therefor, a coupling tube between each antenna and the input of said receiving apparatus, a trigger'circuit for supply.

ing bias potentials to said coupling tubes to ren der thcm alternately conductive, a control .tube

apparatus, of a second fading control system and associated, apparatus same in construction to said first system, and means for supplying the alternating current outputs of said first and second-fading control systems to the inputs of still a third fading control system having the same construction as said first systems, said third fading control system differing from said first and second systems solely in lacking antennas.

9. The combination with a first fading control system comprising a pair of antennas having relatively different fading characteristics, radio receiving apparatus therefor including a detector,

acoupling tube between each antenna and the input of said radio receiving` apparatus, a trigger circuit for supplying bias potentials to said coupling tubes to' render them alternately conductive, a control tube for controlling said trigger circuit, said control tube having an electrode y structure, a source of low frequency currents coupled to an electrode of said control tube to enable said control tube 'to pass current at the frequency of said source, and means for controlling the conductivity of said control tube from the detected output of said radio receiving apparatus, of a third antenna, and means for supplying the output of said first system and energy from said third antenna to a selecting system, said selecting system having apparatus for selecting signals from either said first system or from said third antenna.l y

10. A fading control system comprising a pair of wave receiving structures having relatively different fading characteristics, radio receiving apparatus therefor, a coupling tube betweeneach wave receiving structures and the input of said receiving apparatus, an electronic control circuit having two degrees of electrical stability for supplying bias potentials to said coupling tubes to render them alternatively conductive, a control tube for controlling said electronic control circuit, said control -tube including 1 a control electrode, a source-of low frequency currents of the order of sixty `cycles per second coupled to said electrode of said control tube and supplying voltage thereto of such magnitude and polarity as to enable said control tube to pass. current at the frequency of saidsource, and rectifier means coupled to the output of said radio receiving apparatus and to said control tube for producing a negative voltage of such value as tocut off the yflow of current through said control tube in the presence of a strong incoming signal, said source of low frequency currents exercising control of said conftrol tube whenever the negative voltage produced-by said rectifier falls below a predetermined value as aresult of reduced intensity of incoming signal.

11. .A high frequency system comprising a common transmission medium for signals, radio receiving apparatus including a rectifier, separate paths couplingv said medium to the input of said radio receiving apparatus, a couplingelectron dischargedevice in each of said paths, and means for rendering only that path conductive upon kwhich is impressed a strong signal, said means including a multi-grid vacuum tube, a circuit for controlling the conductivity of said multi-grid tube from the rectified output of said radio receiving apparatus, said circuit being coupled to a grid of said multi-grid tube other than said first grid, a source of low frequency current coupled tothe first grid of said multi-grid tube and an electronic trigger circuit having two degrees of electrical stability under the control of the output of said multi-grid tube for supplying different bias potentials to said coupling devices. said trigger circuit comprising a pair of interconnected electrode structures.

12. A fadingI control system comprising a pair of wave receiving structures having relatively different fading characteristics, radio frequency receiving apparatus therefor including a rectifier, a coupling tube between each wave receiving structure and the input of said receiving apparatus, an electronic control circuit having two degrees of electrical stability for supplying bias potentials to said coupling tubes to render them alternatively conductive, a control tube for controlling said electronic control circuit, said con@ trol tube having an electrode structure, a source of low frequency currents of the order of 60 cycles per second coupled to an electrode of said control tube to enable said control tube to pass current at the frequency. of -said source, a connection from said rectifier to said control tube, said rectifier producing a negative voltage on said connection to cut off the flow of current through said control tube in the presence of a strong incoming signal, said source of low frequency currents exercising control'of said cpntrol tube whenever the negative voltage produced by said rectifier falls below a predetermined value as a result of reduced intensity of kincoming A signal.

13..A receiving system comprising radio frequency receiving apparatus including a detector and having a pair of input circuits each having an electron discharge device therein, means including a source of low audio frequency'alternating current for rendering said devices alternately conductive, and means responsive to the detectedoutput voltage from s aid receiving apparatus exceeding -a predetermined value for removing the control of said source over said devices from said first means.

14. A receiving system comprising radio frequency receiving apparatus having a plurality of input circuits, each of said input circuits including an evacuated electron discharge device, an electronic circuit of two degrees of electrical stability and having -a pair of outputs coupledv to said devices for rendering them alternately conductive. said receiving apparatus including a rectifier, means coupled between the output of said rectifier and the input of said electronic circuit and responsive to a-predetermined magnitude of rectified voltage for controlling the stable position of said electronic circuit and hence the 10 conductive condition of said devices, and a. source of low audio frequency alternating current UNITED STATES PATENTS coupled to the input of said electronic circuit for Number Name Date controlling the stable position thereof in the ab- 1,913,428 Bruce June 13, 1933 sence of said predetermined magnitude of recti- 5 1,959,281 Willoughby May 15, 1934 ed voltage. 2,189,317 Koch Feb. 6, 1940 MURRAY G. CROSBY. 2,243,118 Peterson May 27, 1941 2,282,526 Moore May 12, 1942 REFERENCES CITED 2,290,992 Peterson July 2a, 1942 The following references are of record in the 10 le of this patent: 

